Why on earth do you program with hard coded numbers like 0x07? Kabasan has already showed you that there are symbolic names for such bit patterns like PMIC_HILVLEX_bm. In fact when you look at an Xmega device header you find:

Your code is enabling the TXC interrupt. Do you have ISR(USARTD1_TXC_vect) defined? The default compiler behavior is to execute the reset vector for undefined ISRs. The code you've share will transmit a byte, reset, transmit a byte, reset, transmit a byte.... Since it appears you're debugging via blinkenlights, toggle the LED at the beginning of main() to see if this is happening in your full code.